An automated biosensor system (BIAcore) designed for measuring molecular interactions in real time and without labelling any of the reactants was used for mapping the epitopes of tobacco mosaic virus protein using conformationally specific monoclonal antibodies (MAbs). Some of the MAbs used as capturing antibody on the sensor chip allowed a conformational change to occur in the viral protein. As a result, MAbs specific for the quaternary structure of polymerized viral protein were able to bind to monomeric viral subunits. Compared with classical solid-phase enzyme immunoassay, the biosensor technology possesses several advantages for epitope mapping of viral proteins.